Protection-switching method in asynchronous transfer mode system with ring topology

ABSTRACT

The present invention discloses a protection-switching method in an asynchronous transfer mode ATM system with ring topology, with the protection-switching method of an ONU including the ONU detecting a trouble, or abnormal states, of an optical path connected with the ONU itself, the ONU performing the protection-switching function receiving an ATM cell from the opposite direction of the optical path having the trouble, the software unit of the ONU inserting a node identification data NID of the ONU to the APS register of the optical path of normal states, and the hardware unit of the ONU generating the APS cell by performing mirroring of the APS register to the VCI area of the APS cell, and transmitting the APS cell to the same direction with the direction of the optical path of an abnormal states. Therefore, a protection-switching method in an ATM system with ring topology of the present invention has an advantage of reducing a switching delay time and a switching load.

CLAIM OF PRIORITY

[0001] This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from an application for PROTECTION SWITCHING METHOD OF ATM SYSTEM HAVING A RING CONFIGURATION earlier filed in the Korean Industrial Property Office on Nov. 29, 2001 and there duly assigned Serial No. 2001-74986.

BACKGROUND OF THE INVENTION

[0002] 1. Field of the Invention

[0003] The present invention relates to an asynchronous transfer mode ATM system with ring topology, and more particularly, to a protection-switching method to an ATM cell of the ATM system.

[0004] 2. Description of the Related Art

[0005] Recently, a communication service is upgraded from a voice service, such as a telephone, to a data service, such as Internet, videophone, video on demand VOD, and so on.

[0006] Because the conventional network, such as an integrated service digital network ISDN or a leased line, used in the conventional data service allocates a specified frequency to a subscriber forcedly, it is difficult to apply the conventional network to a recent data communication service, such as Internet, requesting an intermittent and a little data flow. Therefore, an asynchronous transfer mode ATM system has been requested. One of the ATM systems is a fiber loop carrier—C FLC-C system developed by the lead of Korea Telecom KT.

[0007] The FLC-C includes a host digital terminal HDT and numerous optical network units ONUs with star topology, and performs protection-switching function to an optical fiber line. To be applied to service area and situations, the FLC-C is improved to ring topology and to double star topology. However, up to these days, the HDT and the ONU with ring topology is only used in a communication method of a synchronous digital hierarchy SDH. Therefore, for the ATM method, not only is an international standard not prepared completely, but also a commercial product does not exist.

[0008] The general protection-switching method is described in Korean patent application No. 1999-23730 titled by “Protection-Switching Method in Asynchronous Transfer Mode With Ring Topology”, dated on Jun. 23, 1999, by the inventor of this application, in detail.

[0009] The conventional FLC-C system with ring topology depends on software program to detect, insert and switch in protection the APS cell entirely. Therefore, the FLC-C system has a problem that the process time is delayed.

SUMMARY OF THE INVENTION

[0010] It is therefore an object of the present invention to provide protection-switching method in an asynchronous transfer mode ATM system with ring topology, generating, detecting, inserting, and switching in protection an ATM cell in both software and hardware. Therefore, the method of the present invention has a simple structure, and reduces a delay time.

[0011] It is another object to provide an ATM system with ring topology to perform protection-switching operation by referring the data inserted in the APS cell only in order for a load for performing protection-switching operation to be reduced.

[0012] To overcome the above described and other problems and to satisfy the above and other objects, preferred embodiments of the present invention provide a protection-switching method in an asynchronous transfer mode ATM system with ring topology, using an auto protection switching APS cell including a virtual channel identifier VCI to store a data of an optical path having abnormal or normal states, in the case that the ATM system transmitting the same ATM cell to both direction is connected in ring shape with a host digital terminal HDT and numerous optical network units ONUs, wherein each of the ONUs includes: a hardware unit; a software unit, both the hardware and the software unit generating the APS cell; and an APS register which mirroring of the VCI area of the APS cell from/to is performed by the hardware unit and which is read and set by the software unit, and wherein the protection-switching method of an ONU includes: the step that the ONU detects a trouble, or abnormal states, of an optical path connected with the ONU itself; the step that the ONU performs the protection-switching function receiving an ATM cell from the opposite direction of the optical path having the trouble; the step that the software unit of the ONU inserts a node identification data NID of the ONU to the APS register of the optical path of normal states; and the step that the hardware unit of the ONU generates the APS cell by performing mirroring of the APS register to the VCI area of the APS cell, and transmits the APS cell to the same direction with the direction of the optical path of an abnormal states.

[0013] In addition, the step of detecting abnormal states of an optical path includes: the step that the hardware unit of the ONU detects the optical signal of the optical path, generates an interrupt in the case that the optical signal is cut down, and informs the abnormal states of the optical path to the software unit of the ONU; and the step that the software unit of the ONU detects the intensity of the optical signal of the optical path and abnormal states of the optical path in the case that the intensity of the optical signal is lower than the critical value of the intensity.

[0014] In addition, the step of transmitting the APS cell by the hardware unit of the ONU includes that the hardware unit of the ONU transmits the APS cell in periodic interval, until a message of protection-switching completion is inputted from the HDT and the other ONUs.

[0015] Another object of the present invention is to provide a protection-switching method in an ATM system with ring topology, using an auto protection switching APS cell including a virtual channel identifier VCI to store a data of an optical path having abnormal or normal states, in the case that the ATM system transmitting the same ATM cell to both direction is connected in ring shape with a host digital terminal HDT and numerous optical network units ONUs, wherein each of the ONUs includes: a hardware unit; a software unit, both the hardware and the software unit generating the APS cell; and an APS register which mirroring of the VCI area of the APS cell from/to is performed by the hardware unit and which is read and set by the software unit, and wherein the protection-switching method of an ONU includes: the step that the hardware unit of the ONU receives the APS cell to perform mirroring the VCI area of the APS cell to the APS register; the step that the software unit of the ONU checks whether the APS cell is an alarm indication signal AIS or not; the step that the protection-switching function is performed to receive the APS cell of an ONU, having abnormal optical path indicated in the VCI area of the APS cell, from the opposite direction of the optical path in the case that the APS cell is the AIS; the step that the software unit of the ONU inserts a node identification data NID of the ONU to the optical path having the APS register of normal states in the case that the APS cell is the AIS; and the step that the hardware unit of the ONU performs mirroring of the APS register to the VCI area of the APS cell to generate the APS cell again, and transmit the APS cell to the direction of the optical path.

[0016] In addition, the protection-switching method in the ATM system with ring topology further includes that the hardware unit of the ONU performs mirroring of the APS cell to the APS register, generates an interrupt, and informs the received data of the APS cell to the software unit of the ONU sequentially.

[0017] In addition, the step of checking whether the APS cell is the AIS or not, by the software unit of the ONU, includes the step that the software unit of the ONU examines the bit of the identification position of a node transmitting the APS cell among all bits of the APS register to decide whether the APS cell is the AIS or not.

[0018] Another object of the present invention is to provide a protection-switching method in an ATM system with ring topology, using an auto protection switching APS cell including a virtual channel identifier VCI to store a data of an optical path having abnormal or normal states, in the case that the ATM system transmitting the same ATM cell to both direction is connected in ring shape with a host digital terminal HDT and numerous optical network units ONUs, wherein the HDT includes: a hardware unit; a software unit, both the hardware and the software unit detecting the APS cell and generating the APS clear cell; and an APS register which mirroring of the VCI area of the APS cell from/to mutually is performed by the hardware unit and which is read and set by the software unit, and wherein the protection-switching method of the HDT includes: the step that the HDT detects abnormal states of the optical path connected with the HDT; the step that the HDT performs the protection-switching function receiving the ATM cell from the opposite direction of the optical path faced with the abnormal states; the step that the hardware unit of the HDT receives the APS cell to perform mirroring of the VCI area of the APS cell to the APS register; the step that the software unit of the HDT checks whether the APS cell is an AIS or not; and the step that the protection-switching function is performed to receive the APS cell of an ONU, having abnormal optical path indicated in the VCI area of the APS cell, from the opposite direction of the optical path in the case that the APS cell is the AIS;

[0019] In addition, the protection-switching method in the ATM system with ring topology, in the case of the APS clear cell generation situations, further includes: the step that all of the bits in the APS register are reset; the step that the hardware unit of the HDT performs mirroring of the APS register to the VCI area of the APS cell; the step that the hardware unit of the HDT generates the APS clear cell; and the step that the hardware unit of the HDT transmits the APS clear cell to the ONUs.

[0020] In addition, the step of detecting abnormal states of the optical path by HDT includes: the step that the hardware unit of the HDT detects the optical signal of the optical path, generates an interrupt in the case that the optical signal is cut down, and informs the abnormal states of the optical path to the software unit of the HDT; and the step that the software unit of the HDT detects the intensity of the optical signal of the optical path, and abnormal states of the optical path in the case that the intensity of the optical signal is lower than the critical value of the intensity.

[0021] In addition, the protection-switching method in the ATM system with ring topology further includes the hardware unit of the HDT performs mirroring of the APS cell to the APS register, generates an interrupt, and informs the received data of the APS cell to the software unit of the HDT sequentially.

[0022] In addition, the step of checking whether the APS cell is the AIS or not, by the software unit of the HDT, includes the step that the software unit of the HDT examines the bit of the identification position of a node transmitting the APS cell among all bits of the APS register to decide whether the APS cell is the AIS or not.

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] A more complete appreciation of the invention, and many of the attendant advantages thereof, will be readily apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols indicate the same or similar components, wherein:

[0024]FIG. 1 is a schematic diagram of the conventional asynchronous transfer mode ATM system, Fiber Loop Carrier—C FLC-C, with ring topology;

[0025]FIG. 2 is a configuration of an auto protection-switching APS cell;

[0026]FIG. 3 is a schematic diagram of an optical network interface unit ONIU in a second optical network unit ONU according to the preferred embodiment of the present invention;

[0027]FIG. 4 is a flow chart illustrating the APS cell generation procedure in a first field programmable gate array FPGA of the second ONU according to the present invention;

[0028]FIG. 5 is a flow chart illustrating a detection and insertion procedure for the APS cell in the first FPGA of the second ONU according to the present invention;

[0029]FIG. 6 is a flow chart illustrating a protection-switching procedure in a controller of the second ONU according to the present invention;

[0030]FIG. 7 is a schematic diagram of an ONIU in a host digital terminal HDT according to the present invention; and

[0031]FIG. 8 is a flow chart illustrating an APS clear cell generation procedure in a field programmable gate array mFPGA of the HDT according to the present invention.

DETAILED DESCRIPTION OF PREFFERED EMBODIMENTS

[0032]FIG. 1 is a schematic diagram of the conventional ATM system, the FLC-C, with ring topology. HDT 10 and four ONUs 11, 12, 13 and 14 are in a network with ring topology, and communicate through an ATM method. The FLC-C system can include one HDT and a maximum of 14 ONUs and one reserved node. For the convenience of explanation, this application illustrates an FLC-C including 4 ONUs. However, the present invention is not restricted to this case.

[0033] The HDT 10 and the ONUs 11, 12, 13, and 14 transmit an ATM cell through a first and a second direction of an optical fiber line 16 simultaneously, while receiving the ATM cell through either the first direction 161 or the second direction 162. When an arbitrary point has a trouble in an optical fiber line 16, the optical fiber line 16 having the trouble is switched, and the other optical fiber line 16 is not switched.

[0034] For example, when an optical path ‘1 Dr’ in the first direction 161 of the optical fiber line has a trouble, while the optical path ‘1 Dr’ transmits an ATM cell of a first ONU 11 to a second ONU 12, the trouble is detected by the second ONU 12. When detecting the trouble in the optical path ‘1 Dr’, the second ONU 12 performs switching function receiving the ATM cell through the second direction 162. Therefore, the second ONU 12 receives the ATM cell of the first ONU 11 from the second direction 162 passing through the HDT 10, a fourth ONU 14 and a third ONU 13.

[0035] Additionally, when the optical path ‘1 Dr’ is in trouble, the second ONU 12 has to announce the trouble to the other nodes, such as the HDT, and the third and the fourth ONUs. Therefore, the other nodes receive the ATM cell transmitted in the second direction 162 only, while receiving no ATM cell through the first direction 161.

[0036] For performing the above described operation, the second ONU 12 transmits an auto protection-switching APS cell, having a node identification data NID of the second ONU 12, through the first direction 161. The APS cell includes a data of an optical path having no trouble. Therefore, when the other node receives the APS cell having an NID of a node through the first direction 161, the optical path is normally operated from the node to the other node through the first direction 161.

[0037] Therefore, when the second ONU 12 transmits an APS cell through the first direction 161, the third ONU 13 receives the APS cell, and performs protection-switching function according to the NID in the APS cell. In other words, the APS cell includes the NID of the second ONU 12 only. Therefore, through the first direction 161, the third ONU 13 receives an ATM cell from the second ONU 12 only. In addition, the third ONU 13 receives an ATM cell from another node, such as the HDT or the other ONUs, through the second direction 162 only.

[0038] While performing protection-switching function, the third ONU inserts its own NID to the APS cell and transmits the APS cell through the first direction 161 to inform to the fourth ONU 14. The APS cell received by the fourth ONU 14 includes the NID of the second ONU 12 and the third ONU 13. Therefore, through the first direction 161, the fourth ONU 14 receives an ATM cell from the second and the third ONUs 12, 13 only. In addition, the fourth ONU 14 receives an ATM cell from the other node, such as HDT or the other ONUs through the second direction 162.

[0039] In this way, the HDT 10 and the ONUs 12, 13, and 14 perform protection-switching function receiving an ATM cell through the second direction 162 only, even though the ATM cell is transmitted to the first direction 161 having the troubled optical path ‘1 Dr’ at the beginning. In addition, the HDT 10 and the ONUs 11,12, 13, and 14 do not perform protection-switching function to an ATM cell, when the ATM cell does not path through the optical path ‘1 Dr’ of the first direction 161.

[0040] As described in the above statements, each of the HDT 10 and the ONUs 11, 12, 13, and 14 switches in protection to receive an ATM cell transmitted from the opposite direction to a troubled optical path. In addition, each of the HDT 10 and the ONUs 11, 12, 13, and 14 generates an APS cell with its own NID, and transmits the APS cell to the direction of the troubled optical path.

[0041] In addition, each of the HDT 10 and the ONUs 11, 12, 13, and 14 inserts its own NID to the APS cell, and transmits the APS cell continuously to the transmission direction of the APS cell. Therefore, each of the HDT 10 and the ONUs 11, 12, 13, and 14, does not perform protection-switching function to an ATM cell from a node, when the NID of the node is included in the APS cell. On the contrary, each of the HDT 10 and the ONUs 11, 12, 13, and 14, performs protection-switching function to an ATM cell from a node, when the NID of the node is not included in the APS cell to receive the ATM cell from the opposite direction of the optical fiber line.

[0042] Reference will now be made in detail to preferred embodiments of the present invention, example of which is illustrated in the accompanying drawings.

[0043] The asynchronous transfer mode ATM system of the present invention is the same with the ATM system of FIG. 1 with however the differences as shown below.

[0044] In an auto protection-switching APS cell, a specified generic flow control and virtual path identifier GFC/VPI is determined, and a 16-bit virtual channel identifier VCI designates a NID of a node.

[0045]FIG. 2 is a configuration of an APS cell. The APS cell includes an area of a GFC, a VPI, a VCI, and a payload type identifier PTI. The GFC and the VPI area use 0*FF. Each bit in the 16 bits VCI area is inserted by an NID of a node, such as a HDT or an ONU, in a normal optical path. In other words, in the 16-bit VCI area, the lowest bit includes an NID for the HDT, and the bit in the middle includes an NID for each of the corresponding ONU. In addition, the highest bit of the 16-bit VCI area includes an NID for a reserved node.

[0046] Because the protection-switching method is the same among the ONUs 11,12,13 and 14, the second ONU 12 will be an example describing the inner structure and the protection-switching method in detail in the following statement.

[0047] When an input optical signal is cut down (signal fail), the intensity of the signal is lowered than the critical value (signal degrade), and an operator cuts down the signal in arbitrary, a protection-switching function is performed. In ring topology, each node can perform a switching function through the APS cell. When a trouble happens to a physical optical fiber line 16, each node generates an APS cell and transmits the APS cell to an adjacent node in each predetermined period.

[0048] In the present invention, the generation of the APS cell is performed in software, and the periodic transmission of the generated APS cell is performed in hardware. Therefore, the software process is minimized to reduce a delay time. In addition, when the APS cell is inputted from the adjacent node, each node detects, inserts, and switches in protection the APS cell in software and hardware concurrently to reduce a delay time.

[0049]FIG. 3 is a schematic diagram of an optical network interface unit ONIU in a second ONU of the present invention. Referring to FIG. 3, the ONIU of the second ONU includes a first network ATM unit NAU 310, a second NAU 320, an asymmetric digital subscriber line ADSL terminal unit. ATU 330, and an ONU main control unit, controller, 340.

[0050] Each NAU includes a user network interface UNI, a field programmable gate array FPGA, an automatic testing and load analysis system ATLAS, and two cell routers CRa and CRb.

[0051] The first UNI 311 of the first NAU 310 receives an optical signal of the first direction 161 from a first optical fiber line 16, the first optical path, and extracts an ATM cell to transmit the ATM cell to the first FPGA 312. In addition, the first UNI 311 detects states of the optical signal from the first optical path to transmit the states of the optical signal to the first FPGA 312. In addition, the first UNI 311 converts the ATM cell transmitted from the first FPGA 312 to an optical signal, and transmits the optical signal to the second direction 162 of the optical fiber line 16 through the second optical path.

[0052] The first FPGA 312 includes a software unit and a hardware unit. The first FPGA 312 informs a trouble of an optical path detected by the first UNI 311 to the controller 340. Sequentially, the first FPGA 312 generates an APS cell inserted by its own NID, and transmits the APS cell to a third optical path through the second NAU 320. Additionally, following the protection-switching order of the controller 340, the first FPGA 312 transmits an ATM cell from the first UNI 311 to the ATU 330 through the first ATLAS 313 and the first CRb 315, or discards the ATM cell.

[0053] In addition, when an APS cell is inputted form the second NAU 320, the first FPGA 312 transmits the APS cell to the second optical path through the first UNI 311. Additionally, when an APS cell is inputted through the first UNI 311, the first FPGA 312 transmits the APS cell to the controller 340. After inserting its own NID to the APS cell in addition, the first FPGA 312 transmits the APS cell to the third optical path through the second NAU 320. The operation of the first FPGA 312 is later described in more detail in this application.

[0054] The first ATLAS 313, managing an operation, administration, and maintenance OAM of the network, receives an ATM cell dropped into the corresponding node through the first FPGA 312, and transmits the ATM cell to the ATU 330 through the first CRb 315. In addition, the first ATLAS 313 receives an ATM cell added in the corresponding node through the ATU 330 and the first CRb 315, and transmits the ATM cell to the first FPGA 312. In this case, the ATM cell added in the corresponding node, is inputted to the first CRb 315 and the second CRb 325 simultaneously. Therefore, the ATM cell is transmitted to the both directions concurrently.

[0055] The first CRa 314 performs cell routing function transmitting an ATM cell passing through the corresponding node to the second NAU 320 and the second CRa 324. Similarly, the second CRb 315 performs cell routing function receiving an ATM cell added to the corresponding node through the ATU 330, and transmitting an ATM cell dropped in the corresponding node to the ATU 330.

[0056] The ATU 330 receives an ATM cell transmitted from the network to the subscriber through the first CRb 315 and the second CRb 325. The received ATM cell is transmitted to the numerous ADSL modems by using a level two in a universal test & operation PHY (physical or physical layer protocol) interface for ATM (UTOPIA). Sequentially, to transmit data in high speed by using a telephone line, the ADSL modem modulates the corresponding ATM cell to an analog signal, and transmits the ATM cell to a subscriber. On the contrary, after demodulating an analog signal from a subscriber to a digital signal, the ATM cell is transmitted to a first CRb 315 and the second CRb 325 by using the level two in UTOPIA.

[0057] Additionally, the second UNI 321 of the second NAU 320 receives an optical signal from the fourth optical path, extracts an ATM cell from the optical signal, and transmits the ATM cell to the second FPGA 322. As a result, the ATM cell is transmitted to the second direction 162 of the optical fiber line. In addition, the second UNI 321 detects the states of the optical signal, and informs the states of the optical signal to the second FPGA 322. In addition, the second UNI 321 converts an ATM cell from the second FPGA 322 to an optical signal, and transmits the optical signal to the first direction 161 of the optical fiber line 16 through the third optical path.

[0058] The second FPGA 322 includes a software unit and a hardware unit. When detecting a trouble in an optical fiber line, the second FPGA 322 informs the trouble to the controller 340, and generates an APS cell inserted by its own NID to transmit the APS cell to the second optical path through the first NAU 310. In addition, following the protection-switching order from the controller 340, the second FPGA 322 transmits an ATM cell of the second UNI 321 to the ATU 330 through the second ATLAS 323 and the second CRb 325, or discards the ATM cell. In addition, when the APS cell is inputted from the first NAU 310, the second FPGA 322 transmits the APS cell to the third optical path through the second UNI 321. Additionally, when an APS cell is inputted through the second UNI 321, the second FPGA 322 transmits the APS cell to the controller 340, and inserts its own NID to the APS cell in addition to transmit the APS cell to the second optical path through the first NAU 310. The operation of the second FPGA 322 is described later in more detail in this application.

[0059] The second ATLAS 323 managing OAM of the network, receives an ATM cell dropped into the corresponding node through the second FPGA 322, and transmits the ATM cell to the ATU 330 through the second CRb 325. In addition, the second ATLAS 323 receives an ATM cell added from the corresponding node through the ATU 330 and the second CRb 325, and transmits the ATM cell to the second FPGA 322. The second CRa 324 performs cell routing function transmitting an ATM cell passing through the corresponding node to the first CRa 314 of the first NAU 310. In addition, the second CRb performs cell routing function transmitting an ATM cell dropped into the corresponding node to the ATU 330, and receiving an ATM cell added from the corresponding node from the ATU 330.

[0060] The controller 340 not only controls the first NAU 310 and the second NAU 320 entirely, but also performs protection-switching function, according to the trouble data and an APS cell of the first FPGA 312 and the second FPGA 322. Additionally, because the first FPGA 312 and the second FPGA 322 is directly connected with the controller 340, an APS cell frame and clock data are directly transmitted from/to (from or to or both from and to) the controller 340.

[0061] Now, the operation generating, detecting, and inserting an APS cell in the first FPGA 312 and the second FPGA 322 in the second ONU 12, and the protection-switching operation of the controller 340 in the second ONU 12 will be explained in referring to flow charts from FIG. 4 to FIG. 6.

[0062]FIG. 4 is a flow chart illustrating the APS cell generation procedure in the first FPGA 312 of the second ONU 12 of the present invention.

[0063] The hardware unit of the first FPGA 312 detects an optical signal received from the first optical path (step S401). When the optical signal is cut down by the disconnection of the first optical path or by other reasons (step S402), abnormal states have been generated. Therefore, the hardware unit generates an interrupt to report the abnormal states of the first optical path to the software unit (step S403), and the step S406 is progressed.

[0064] However, when the optical signal is not cut down (step S402), the software unit of the first FPGA 312 checks the intensity of the corresponding optical signal (step S404). As a result, according to the trouble of the first optical path, when the intensity of the corresponding optical signal is lower than the critical value (step S405), the first optical path has a trouble. Therefore, the step S406 is progressed. However, when the intensity of the optical signal is higher than the critical value (step S405), the first optical path is in normal states. Therefore, the step S401 is progressed and waits for the next optical signal.

[0065] When the first optical path is in trouble, the software unit informs of the abnormal states to the controller so that the controller performs protection-switching function (step S406). In addition, after the bit of the NID in the outputted APS register is set to ‘1’, the APS register is transmitted to the hardware unit (step S407). The hardware unit performs mirroring of the outputted APS register to the VCI area of the APS cell to generate an APS cell (step S408), and transmits the generated APS cell in periodic interval, such as 1, 2, 5, 10, or 20 mSec (millisecond), to the third optical path through the second NAU 320 (step S409). When a message of “completion of protection-switching operation in all of the nodes” is inputted, the hardware unit does not transmit the APS cell any more.

[0066]FIG. 5 is a flow chart illustrating a detection and insertion procedure for the APS cell in the first FPGA of the second ONU according to the present invention.

[0067] The hardware unit of the first FPGA 312 detects the APS cell from the first optical path (step S501). When the event happens (step S502), the VCI area of the APS cell is mirrored to an input APS register, an interrupt is generated, and the input APS cell is reported to the software unit (step S503). Sequentially, the software unit inspects the input APS register (step S504) and reviews whether the corresponding APS cell is an alarm indication signal AIS or not (step S505). At this moment, all bits of the input APS register are not examined. Instead, an identifier position of a node connected with the first optical path is examined. In other words, in the case of the second ONU 12, because the first optical path is connected with the first ONU 11, the position of ‘1h’ is examined. When the corresponding bit is set, the APS cell is considered as an AIS signal.

[0068] When the APS cell is an AIS signal (step S505), the software unit transmits the input APS cell to the controller 340 (step S506). In addition, after setting the bit of the NID position, the APS cell is transmitted to an output APS register of the second FPGA 322 (step S507).

[0069] The software unit of the second FPGA 322 performs mirroring of the contents of the output APS register to the VCI area of the APS cell transmitted to the second UNI 321. On the contrary, when an APS cell is generated to the second direction 162 and an input APS register data of the second FPGA 322 is transmitted to an output APS register of the first FPGA 312, the software unit of the first FPGA 312 performs mirroring of the contents of the output APS register to the VCI area of the APS cell transmitted to the first UNI 311. When the APS cell is not an AIS signal (step S505), step S501 is then repeated.

[0070]FIG. 6 is a flow chart illustrating a protection-switching procedure in a controller 340 of the second ONU 12 according to the present invention.

[0071] When the first FPGA 312 informs the abnormal states of the first optical path (step S601), the controller 340 checks whether switching to the fourth optical path is possible or not, and performs protection-switching function for all of the ATM cells to receive only an ATM cell from the fourth optical path (step S602).

[0072] When the abnormal states in the first optical path is not detected in the step S601, or protection-switching function is completed in the step S602, the controller 340 performs a step S603. When the second FPGA 322 informs the abnormal states of the fourth optical path (step S603), the controller 340 checks whether switching to the first optical path is possible or not, and performs protection-switching function for all of the ATM cells to receive only an ATM cell from the first optical path (step S604).

[0073] When the abnormal states in the fourth optical path is not detected in the step S603, or protection-switching function is completed in the step S604, the controller 340 performs a step S605. When receiving an APS cell from the first FPGA 312 through the first optical path (step S605), the controller 340 analyzes the received APS cell, checks whether switching to the fourth optical path is possible or not, does not perform protection-switching function to an ATM cell from a node that an NID of the node is inserted into the received APS cell, and perform protection-switching function receiving only an ATM cell from the fourth optical path to a node that an NID of the node is not inserted in the received APS cell (step S606).

[0074] When an APS cell is not transmitted from the first FPGA 312 in the step S605, or protection-switching function is completed in the step S606, the controller 340 performs a step S607. When receiving an APS cell from the second FPGA 322 through the second optical path (step S607), the controller 340 analyzes the received APS cell, checks whether switching to the first optical path is possible or not, does not perform protection-switching function to an ATM cell from a node that an NID of the node is inserted into the received APS cell, and performs protection-switching function receiving only an ATM cell from the first optical path to a node that an NID of the node is not inserted in the received APS cell (step S608).

[0075] From now on, the operation for protection-switching in the HDT 10 is illustrated. The HDT 10 differing from the ONU, is a node terminating the APS cell and generating an APS clear cell. However, the HDT 10 also generates an APS cell, wherein a software unit generates an APS cell and a hardware unit donates the generated APS cell in periodic interval, such as 10, 20, or 100 mSec.

[0076]FIG. 7 is a schematic diagram of an ONIU in the HDT 10 according to the present invention. Even though the ATM FLC-C system with ring topology of FIG. 1 has one HDT with one ring, the ONIU of one HDT can be connected with multiple rings including numerous ONUs.

[0077] The ONIU of the HDT includes NAUa 710, NAUb 720, ATU 730, and a HDT main control unit HMCU 740 in large. The NAUa 710 includes a first mUNI 711, a first mFPGA 712, and a first mATLAS 713, and the NAUb 720 includes a second mUNI 721, a second mFPGA 722,and a second mATLAS 723.

[0078] The first mUNI 711 of the NAUa 710 receives an optical signal of the first direction 161 from the optical path ‘A’, extracts an ATM cell from the optical signal, and transmits the ATM cell to the first mFPGA 712. In addition, the first mUNI 711 detects the states of the optical path ‘A’, and informs the data of the states to the first mFPGA 712. In addition, the first mUNI 711 converts an ATM cell of the first mFPGA 712 to an optical signal, and transmits the optical signal to the second direction 162 through the optical path ‘B’.

[0079] The first mFPGA 712 includes a software unit and a hardware unit. The first mFPGA 712 provides the HMCU 740 with abnormal states data of the optical path ‘A’ detected by the first mUNI 711. In addition, following the protection-switching order of the HMCU 740, the first mFPGA 712 transmits an ATM cell of the first mUNI 711 to the ATU 730 through the first MATLAS 713, or discards the ATM cell. When the first mUNI 711 transmits an APS cell, the first mFPGA 712 stores the APS cell and transmits the APS cell to the HMCU 740. Because the HDT 10 is a node terminating the APS cell, the first mFPGA 712 does not transmit the corresponding APS cell to NAUb 720.

[0080] The second mUNI 721 of the NAUb 720 receives an optical signal of the second direction 162 from the optical path ‘D’, extracts an ATM cell from the optical signal, and transmits the ATM cell to the second mFPGA 722. In addition, the second mUNI 721 detects the states of the optical path ‘D’, and informs the data of the states to the second mFPGA 722. In addition, the second mUNI 721 converts an ATM cell of the second mFPGA 722 to an optical signal, and transmits the optical signal to the first direction 161 through the optical path ‘C’.

[0081] The second mFPGA 722 includes a software unit and a hardware unit. The second mFPGA 722 provides the HMCU 740 with abnormal states data of the optical path ‘D’ detected by the second mUNI 721. In addition, following the protection-switching order of the HMCU 740, the second mFPGA 722 transmits an ATM cell of the second mUNI 721 to the ATU 730 through the second mATLAS 723, or discards the ATM cell. When the second mUNI 721 transmits an APS cell, the second mFPGA 722 stores the APS cell and transmits the APS cell to the HMCU 740. Because the HDT 10 is a node terminating the APS cell, the second mFPGA 722 does not transmit the corresponding APS cell to NAUa 710.

[0082] The operation of the first mATLAS 713, the second mATLAS 723, and the ATU 530 are similar with the operation of the first ATLAS, the second ATLAS, and the ATU in an ONIU of an ONU. Therefore, detail explanation will be omitted in this application.

[0083] The HMCU 740 performs protection-switching function, by using states data of the optical path from the first and the second mFPGA 712 and 722, or by using an APS cell from the first and the second mFPGA 712 and 722. The protection-switching procedure of the HMCU 740 of the HDT 10 is the same with that of a controller of an ONU. In other words, when the optical path ‘A’ has trouble, protection-switching function is performed to receive an ATM cell from the optical path ‘D’ only. In the same way, when the optical path ‘D’ has trouble, protection-switching function is performed to receive an ATM cell from the optical path ‘A’ only. In addition, when receiving an APS cell from the first and the second mFPGA 712 and 722, the HMCU 740 does not perform protection-switching function to an ONU, when an NID of the ONU is inserted to the APS cell, and performs protection-switching function to an ONU, when an NID of the ONU is not inserted to the APS cell.

[0084] The operation of the first mFPGA 712 is illustrated in the following statements.

[0085] When an optical signal from the optical path ‘A’ is interrupted, in other words, the optical path ‘A’ is in the abnormal states, the hardware unit of the first mFPGA 712 detects the abnormal states, generates an interrupt, and informs the abnormal states to the software unit. Additionally, when the intensity of the optical signal from the optical path ‘A’ is lower than the critical value, the software unit of the first mFPGA 712 also detects the trouble of the optical path ‘A’. When the optical path ‘A’ is in trouble, the software unit of the first mFPGA 712 informs the trouble of the optical path ‘A’ to the HMCU 740, and the HMCU 740 performs protection-switching function.

[0086] When the hardware unit of the first mFPGA 712 receives an APS cell and performs mirroring of the VCI area of the APS cell to an APS register, the software of the first mFPGA 712 checks the position storing an NID of an ONU transmitting the APS cell. When the bit of the corresponding position is set, the first mFPGA 712 transmits the APS cell to the HMCU 740, and the HMCU 740 performs protection-switching function.

[0087] The operation of the second mFPGA 722 is illustrated in the following statements.

[0088] When an optical signal from the optical path ‘D’ is interrupted, in other words, the optical path ‘D’ is in the abnormal states, the hardware unit of the second mFPGA 722 detects the abnormal states, generates an interrupt, and informs the abnormal states to the software unit. Additionally, when the intensity of the optical signal from the optical path ‘D’ is lower than the critical value, the software unit of the second mFPGA 722 also detects the trouble of the optical path ‘D’. When the optical path ‘D’ is in trouble, the software unit of the second mFPGA 722 informs the trouble of the optical path ‘D’ to the HMCU 740, and the HMCU 740 performs protection-switching function.

[0089] When the hardware unit of the second mFPGA 722 receives an APS cell and performs mirroring of the VCI area of the APS cell to an APS register, the software of the first mFPGA 712 checks the position storing an NID of an ONU transmitting the APS cell. When the bit of the corresponding position is set, the second mFPGA 722 transmits the APS cell to the HMCU 740.

[0090] When the power is reset or the switching procedure is ended, the HDT 10 generates an APS clear cell, and transmits the APS clear cell to ONUs. FIG. 8 is a flow chart illustrating an APS clear cell generation procedure in a mFPGA of the HDT according to the present invention. The mFPGA is a common designation of the first and the second mFPGA 712 and 722 in FIG. 7.

[0091] At first, the software unit of the mFPGA checks the situations whether it is necessary to generate the APS clear cell or not (step S801). When the situations requests generating the APS clear cell (step S802), for example, the power is reset or the switching procedure is ended, the software unit of mFPGA resets all of the bits in the APS register (step S803). Sequentially, the hardware unit of mFPGA performs mirroring the APS register to the VCI area of the APS cell to generate the APS clear cell (step S804), and the generated APS clear cell is transmitted to ONU (step S805). If it is not necessary to generate the APS clear cell (step S802), then step S801 is repeated.

[0092] As described in the above statements, the ATM system with ring topology of the present invention performs protection-switching operation by referring the data inserted in the APS cell only. Therefore, a load for performing protection-switching operation can be reduced. In addition, because the generation, detection, insertion operation in the APS cell is performed in both ways of a software and a hardware, a delay time for protection-switching operation can be reduced. As a result, the ATM system with ring topology of the present invention reduces a switching delay time that can be generated in protection-switching operation, and reduces a load of protection-switching operation.

[0093] While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention. 

What is claimed is:
 1. A protection-switching method in an asynchronous transfer mode system with ring topology, comprising: detecting by the optical network unit for trouble, or abnormal states, of an optical path connected with the optical network unit itself, the asynchronous transfer mode system using an auto protection switching cell including a virtual channel identifier to store a data of an optical path including abnormal or normal states, when the asynchronous transfer mode system transmitting the same asynchronous transfer mode cell to both direction is connected in ring shape with a host digital terminal and a plurality of optical network units, with each of the optical network units comprising: a hardware unit; a software unit, both the hardware and the software unit generating the auto protection switching cell; and an auto protection switching register which mirroring of the virtual channel identifier area of the auto protection switching cell from and to is performed by the hardware unit and which is read and set by the software unit; performing by the optical network unit of the protection-switching function receiving an asynchronous transfer mode cell from the opposite direction of the optical path having the trouble; inserting a node identification data of the optical network unit to the auto protection switching register of the optical path of normal states by the software unit of the optical network unit; and generating, by the hardware unit of the optical network unit, the auto protection switching cell by performing mirroring of the auto protection switching register to the virtual channel identifier area of the auto protection switching cell, and transmitting the auto protection switching cell to the same direction with the direction of the optical path of abnormal states.
 2. The protection-switching method in the asynchronous transfer mode system with ring topology according to claim 1, with the step of detecting abnormal states of an optical path further comprising: detecting, by the hardware unit of the optical network unit, the optical signal of the optical path, generating an interrupt when the optical signal is cut down, and informing the abnormal states of the optical path to the software unit of the optical network unit; and detecting, by the software unit of the optical network unit, the intensity of the optical signal of the optical path and abnormal states of the optical path in the case that the intensity of the optical signal is lower than the critical value of the intensity.
 3. The protection-switching method in the asynchronous transfer mode system with ring topology according to claim 1, with the step of transmitting the auto protection switching cell by the hardware unit of the optical network unit comprises the hardware unit of the optical network unit transmitting the auto protection switching cell in periodic interval, until a message of protection-switching completion is inputted from the host digital terminal and the other optical network units.
 4. A protection-switching method in an asynchronous transfer mode system with ring topology, using an auto protection switching cell including a virtual channel identifier to store a data of an optical path having abnormal or normal states, in the case that the asynchronous transfer mode system transmitting the same asynchronous transfer mode cell to both direction is connected in ring shape with a host digital terminal and numerous optical network units, with each of the optical network units comprising: a hardware unit; a software unit, both the hardware and the software unit generating the auto protection switching cell; and an auto protection switching register where mirroring of the virtual channel identifier area of the auto protection switching cell from and to is performed by the hardware unit and is read and set by the software unit, and with the protection-switching method of an optical network unit comprising: receiving, by the hardware unit of the optical network unit, the auto protection switching cell to perform mirroring of the virtual channel identifier area of the auto protection switching cell to the auto protection switching register; checking, by the software unit of the optical network unit, whether the auto protection switching cell is an alarm indication signal or not; performing the protection-switching function to receive the auto protection switching cell of an optical network unit, including abnormal optical path indicated in the virtual channel identifier area of the auto protection switching cell, from the opposite direction of the optical path when the auto protection switching cell is the alarm indication signal; inserting, by the software unit of the optical network unit, a node identification data of the optical network unit to the optical path including the auto protection switching register of normal states when the auto protection switching cell is the alarm indication signal; and performing, by the hardware unit of the optical network unit, a mirroring of the auto protection switching register to the virtual channel identifier area of the auto protection switching cell to generate the auto protection switching cell again, and transmit the auto protection switching cell to the direction of the optical path.
 5. The protection-switching method in the asynchronous transfer mode system with ring topology according to claim 4, further comprising the hardware unit of the optical network unit performs mirroring of the auto protection switching cell to the auto protection switching register, generating an interrupt, and informing the received data of the auto protection switching cell to the software unit of the optical network unit sequentially.
 6. The protection-switching method in the asynchronous transfer mode system with ring topology according to claim 4, with the step of checking whether the auto protection switching cell is the alarm indication signal or not, by the software unit of the optical network unit, further comprising of examining by the software unit of the optical network unit of the bit of the identification position of a node transmitting the auto protection switching cell among all bits of the auto protection switching register to decide whether the auto protection switching cell is the alarm indication signal or not.
 7. A protection-switching method in an asynchronous transfer mode system with ring topology, using an auto protection switching cell including a virtual channel identifier to store a data of an optical path having abnormal or normal states, when the asynchronous transfer mode system transmitting the same asynchronous transfer mode cell to both direction is connected in ring shape with a host digital terminal and numerous optical network units, with the host digital terminal comprising: a hardware unit; a software unit, both the hardware and the software unit detecting the auto protection switching cell and generating the auto protection switching clear cell; and an auto protection switching register which mirroring of the virtual channel identifier area of the auto protection switching cell from and to mutually is performed by the hardware unit and is read and set by the software unit, and with the protection-switching method of the host digital terminal comprising: detecting, by the host digital terminal, abnormal states of the optical path connected with the host digital terminal; performing, by the host digital terminal, the protection-switching function receiving the asynchronous transfer mode cell from the opposite direction of the optical path faced with the abnormal states; receiving, by the hardware unit of the host digital terminal, the auto protection switching cell to perform mirroring of the virtual channel identifier area of the auto protection switching cell to the auto protection switching register; checking by the software unit of the host digital terminal of whether the auto protection switching cell is an alarm indication signal or not; and performing the protection-switching function to receive the auto protection switching cell of an optical network unit, including abnormal optical path indicated in the virtual channel identifier area of the auto protection switching cell, from the opposite direction of the optical path when the auto protection switching cell is the alarm indication signal.
 8. The protection-switching method in the asynchronous transfer mode system with ring topology according to claim 7, with the auto protection switching clear cell generation, further comprising: resetting all of the bits in the auto protection switching register; performing by the hardware unit of the host digital terminal of mirroring of the auto protection switching register to the virtual channel identifier area of the auto protection switching cell; generating by the hardware unit of the host digital terminal of the auto protection switching clear cell; and transmitting by the hardware unit of the host digital terminal of the auto protection switching clear cell to the optical network units.
 9. The protection-switching method in the asynchronous transfer mode system with ring topology according to claim 7, the step of detecting abnormal states of the optical path by host digital terminal further comprising: detecting, by the hardware unit of the host digital terminal, the optical signal of the optical path, generating an interrupt when the optical signal is cut down, and informing the abnormal states of the optical path to the software unit of the host digital terminal; and detecting, by the software unit of the host digital terminal, the intensity of the optical signal of the optical path, and abnormal states of the optical path when the intensity of the optical signal is lower than the critical value of the intensity.
 10. The protection-switching method in the asynchronous transfer mode system with ring topology according to claim 7, further comprises the hardware unit of the host digital terminal performing mirroring of the auto protection switching cell to the auto protection switching register, generating an interrupt, and informing the received data of the auto protection switching cell to the software unit of the host digital terminal sequentially.
 11. The protection-switching method in the asynchronous transfer mode system with ring topology according to claim 7, with the step of checking whether the auto protection switching cell is the alarm indication signal or not, by the software unit of the host digital terminal, further comprising of examining by software unit of the host digital terminal of the bit of the identification position of a node transmitting the auto protection switching cell among all bits of the auto protection switching register to decide whether the auto protection switching cell is the alarm indication signal or not.
 12. A protection-switching method in an asynchronous transfer mode system with ring topology using an auto protection switching cell, comprising: detecting for trouble, or abnormal states, of an optical path connected with an optical network unit itself; performing by the optical network unit of a protection-switching function receiving an asynchronous transfer mode cell from the opposite direction of an optical path having trouble; inserting a node identification data of the optical network unit to an auto protection switching register of the optical path of normal states by a software unit of the optical network unit; and generating, by a hardware unit of the optical network unit, the auto protection switching cell by performing mirroring of the auto protection switching register to a virtual channel identifier area of the auto protection switching cell, and transmitting the auto protection switching cell to the same direction with the direction of the optical path of abnormal states.
 13. The protection-switching method of claim 12, with the step of detecting abnormal states of an optical path further comprising of detecting, by the hardware unit of the optical network unit, the optical signal of the optical path, generating an interrupt when the optical signal is cut down, and informing the abnormal states of the optical path to the software unit of the optical network unit.
 14. The protection-switching method of claim 13, with the step of detecting abnormal states of an optical path further comprising of detecting, by the software unit of the optical network unit, the intensity of the optical signal of the optical path and abnormal states of the optical path in the case that the intensity of the optical signal is lower than the critical value of the intensity.
 15. The protection-switching method of claim 14, with the step of transmitting the auto protection switching cell by the hardware unit of the optical network unit comprising the hardware unit of the optical network unit transmitting the auto protection switching cell in periodic interval, until a message of protection-switching completion is inputted from the host digital terminal and the other optical network units.
 16. A protection-switching method in an asynchronous transfer mode system with ring topology using an auto protection switching cell, comprising: receiving, by a hardware unit of an optical network unit, the auto protection switching cell to perform mirroring of the virtual channel identifier area of the auto protection switching cell to an auto protection switching register; checking, by a software unit of the optical network unit, whether the auto protection switching cell is an alarm indication signal or not; performing a protection-switching function to receive the auto protection switching cell of the optical network unit, including abnormal optical path indicated in a virtual channel identifier area of the auto protection switching cell, from the opposite direction of the optical path when the auto protection switching cell is the alarm indication signal; inserting, by the software unit of the optical network unit, a node identification data of the optical network unit to the optical path including the auto protection switching register of normal states when the auto protection switching cell is the alarm indication signal; and performing, by the hardware unit of the optical network unit, a mirroring of the auto protection switching register to the virtual channel identifier area of the auto protection switching cell to generate the auto protection switching cell again, and transmit the auto protection switching cell to the direction of the optical path.
 17. The protection-switching method of claim 16, further comprising the hardware unit of the optical network unit performing mirroring of the auto protection switching cell to the auto protection switching register, generating an interrupt, and informing the received data of the auto protection switching cell to the software unit of the optical network unit sequentially.
 18. The protection-switching method of claim 17, with the step of checking whether the auto protection switching cell is the alarm indication signal or not, by the software unit of the optical network unit, further comprising of examining by the software unit of the optical network unit of the data bit of the identification position of a node transmitting the auto protection switching cell among all bits of the auto protection switching register to decide whether the auto protection switching cell is the alarm indication signal or not.
 19. A protection-switching method in an asynchronous transfer mode system with ring topology, using an auto protection switching cell, comprising: detecting abnormal states of the optical path connected with a host digital terminal; performing, by the host digital terminal, a protection-switching function receiving an asynchronous transfer mode cell from the opposite direction of the optical path faced with the abnormal states; receiving, by a hardware unit of the host digital terminal, the auto protection switching cell to perform mirroring of the virtual channel identifier area of the auto protection switching cell to the auto protection switching register; checking by a software unit of the host digital terminal of whether the auto protection switching cell is an alarm indication signal or not; and performing the protection-switching function to receive the auto protection switching cell of an optical network unit, including abnormal optical path indicated in a virtual channel identifier area of the auto protection switching cell, from the opposite direction of the optical path when the auto protection switching cell is the alarm indication signal.
 20. The protection-switching method of claim 19, with the auto protection switching clear cell generation, further comprising: resetting all of the bits in the auto protection switching register; performing by the hardware unit of the host digital terminal of mirroring of the auto protection switching register to the virtual channel identifier area of the auto protection switching cell; and generating by the hardware unit of the host digital terminal of the auto protection switching clear cell.
 21. The protection-switching method of claim 20, with the auto protection switching clear cell generation, further comprising of transmitting by the hardware unit of the host digital terminal of the auto protection switching clear cell to the optical network units.
 22. The protection-switching method of claim 21, with the step of detecting abnormal states of the optical path by host digital terminal further comprising: detecting, by the hardware unit of the host digital terminal, the optical signal of the optical path, generating an interrupt when the optical signal is cut down, and informing the abnormal states of the optical path to the software unit of the host digital terminal; and detecting, by the software unit of the host digital terminal, the intensity of the optical signal of the optical path, and abnormal states of the optical path when the intensity of the optical signal is lower than the critical value of the intensity.
 23. The protection-switching method claim 22, further comprising the hardware unit of the host digital terminal performing mirroring of the auto protection switching cell to the auto protection switching register, generating an interrupt, and informing the received data of the auto protection switching cell to the software unit of the host digital terminal sequentially.
 24. The protection-switching method of claim 23, with the step of checking whether the auto protection switching cell is the alarm indication signal or not, by the software unit of the host digital terminal, further comprising of examining by software unit of the host digital terminal of the bit of the identification position of a node transmitting the auto protection switching cell among all bits of the auto protection switching register to decide whether the auto protection switching cell is the alarm indication signal or not. 